The present invention relates to a flow meter, in which a bluff body of elongated cross-section extends substantially normal to the axis of a conduit through which a fluid, which may be a liquid or a gas, passes, so that vortices are formed in the liquid to opposite sides of the body which alternatingly break away from the latter. The frequency of the alternatingly breaking vortices is proportionate to the speed, respectively the volume per time unit, of the fluid passing through the conduit and this frequency is measured by at least one feeler arranged in a measuring conduit located outside the first-mentioned conduit and in which the outer ends of the measuring conduit communicate through bores with the interior of the first-mentioned conduit to opposite sides of the bluff body.
Such a flow meter has the advantage that the feeler is arranged, not as usual on or in the bluff body and therewith in the mainstream passing through the conduit, but in which the feeler is located in a measuring conduit provided outside of the main conduit. In this way the feeler may be easily exchanged during the operation of the flow meter and in addition it is protected from any mechanical damage, which may be caused by particles taken along by the fluid passing through the main conduit.
In flow meters of the aforementioned kind it is known to arrange the ends of the bores which communicate with the main conduit at the rear region of the bluff body, as considered in the direction of the flow of the fluid flowing through the main conduit, whereby both inner ends are arranged in a plane which is normal to the longitudinal axis of the main conduit, that is, both inner ends are located directly in the region of the leading vortices. The vortices which, according to Karman's street of vortices detach itself alternatingly from the bluff body, will cause at the bores alternating pressure variations, which will produce in the measuring conduit, due to the momentarily produced pressure difference, an alternating fluid stream, the frequency of which is sensed by the feeler and indicated as a measurement for the speed of the fluid passing through the main conduit.
Due to the irregularity of the departing vortices directly in the vortex region, such irregularities are transmitted through the bores also onto the movement of the fluid in the measuring conduit, so that a usable signal output is only obtainable at low frequencies, that is, at a large nominal width of the flow meter. However, at small monimal widths and therewith connected high vortex frequencies, a sure signal reception is not assured, since the irregularities of the movement of the fluid occurring in the measuring conduit, which are due to the not always simultaneously occurring of plus and minus impulses, lead to super impositions and therewith to disturbances in the measuring conduit. In addition it is possible, since the fluid medium in the measuring conduit only pendulates, that during measuring of gas, condensate and during measuring of liquid, gas carried thereby will accumulate in the measuring conduit, so that the signal will be changed or completely extinguished.